I always thought the reason was to leave room for the possibility of a little competition. With a ley so close the bosses would typically just go to the largest clan. Small clans usually bring most everyone to bosses, but monopoly clans often send what they think are enough, but might misjudge sometimes and show up to find themselves outnumbered. With a ley nearby reinforcements can get there quickly enough to outnumber smaller clans if need be, squelching the competition. That's what I was told.